The present invention relates to an ignition system for an internal combustion engine, and more particularly, to an ignition system for an internal combustion engine suitable for use in an integrated control circuit.
Recently, a one-chip ignition system has been developed as an ignition system for an internal combustion engine which has a power IGBT (insulating gate bipolar transistor), a current limiter circuit and an input stage protection circuit fabricated on a monolithic substrate. A conventional known one-chip ignition system, for example, as described in JP-A-9-280147, is configured to supply a small constant current from a collector to a gate by a high voltage constant current device to apply the gate with a voltage when a collector voltage is higher than a gate voltage. This known one-chip ignition system limits oscillations of the collector voltage immediately after starting a current limitation by increasing the gate voltage with the collector voltage increased immediately after starting the current limitation, and suppresses oscillations of the collector voltage during the current limitation by releasing the operation of increasing the gate voltage, when no gate voltage is applied, so as to prevent a gate potential from increasing, while the current is blocked, by the influence of the small constant current.
However, the ignition system for an internal combustion engine described in JP-A-9-280147 has a problem in that a stray current is generated in an actual substrate so that a large current exceeding a value set by a constant current circuit flows through this route to cause an increase in the gate potential. Particularly, when the ignition system comprises a self separation type device, a vertical depletion IGBT is used in the constant current circuit. In the constant current circuit comprising the vertical depletion IGBT formed in self separation in an IGBT substrate, a PNP parasitic transistor exists. A stray current flows into the PNP parasitic transistor, causing an excessive current to flow through a circuit provided for purposes of controlling the gate voltage with a small constant current, with the result that the gate voltage is increased more than necessary. This influence causes a problem of an instable current blocking operation of an ignition coil depending on an ignition system driving method.
It is an object of the present invention to provide an ignition system for an internal combustion engine which allows an ignition coil to perform a stable current blocking operation.
(1) To achieve the above object, the present invention provides an ignition system for an internal combustion engine having a power switching unit for controlling to conduct or block a primary current flowing through an ignition coil in response to an input ignition control signal to generate a high voltage on a secondary side of the ignition coil, a current limiter circuit for limiting the primary current, and a connection circuit for connecting a collector and a gate of the power switching unit, wherein the power switching unit and the current limiter circuit are integrated in a monolithic silicon substrate of an insulating gate bipolar power transistor, the ignition system for an internal combustion engine comprising a path for leading a stray current generated within the monolithic silicon substrate to an emitter of the power switching unit.
With the configuration as described, an unwanted stray is led to the emitter to reduce the influence exerted on the gate, thereby making it possible to stabilize a current blocking operation of the ignition coil.
(2) In the foregoing (1), preferably, the connection circuit comprises an impedance element, and the path comprises a zener diode which operates in accordance with a voltage drop across the impedance element.
(3) In the foregoing (1), preferably, the connection circuit is comprised of a depletion insulating gate bipolar power transistor constituting a constant current circuit formed on the monolithic silicon substrate, and a resistor connected to the depletion insulating gate bipolar power transistor, and the path is comprised of a zener diode connected between the depletion insulating gate bipolar power transistor and the resistor and operative when a voltage generated across the resistor becomes equal or higher than a zener voltage for leading the current to the emitter of the power switching unit.